1. Field of the Invention
The invention relates generally to the field of rotary orientation encoders. More specifically, the invention relates to rotary encoders having high angular resolution and absolute orientation determination capability.
2. Background Art
Rotary encoders provide information about the angular position of rotating device with respect to a selected reference. One type of such encoder uses one or more magnets rotatably mounted with respect to a magnetic field sensor. When the magnet is mounted to a shaft, for example, the sensor provides a signal related to the rotation angle of the shaft with respect to the sensor. There are two type of magnetic rotary encoders, on-axis and off-axis type.
In on-axis magnetic rotary encoders, an example of which is shown at 10 in FIG. 1, a diametrically magnetized magnet 14 is placed on the end of a shaft (not visible in FIG. 1, but affixed to baseplate 12), with a sensor 16 mounted on the coaxially with the shaft and in close proximity to the magnet 14. Analysis of the magnetic field from multiple sense elements within the sensor 16 is used to estimate orientation angle of the magnetic field from the magnet 14 with respect to the sensor 16. On-axis rotary encoders typically provide 8 bits to 12 bits of resolution (28=256 steps per revolution, 212=4096 steps per revolution) and can provide absolute orientation information, i.e., rotary orientation information is directly related to the angle between a line connecting the north to south poles of the magnet 14 and the sensitive axis of the sensor 16. Rotary orientation information is immediately available on application of power to the sensor 16.
In an on-axis sensor, however, the angular resolution however is limited by signal levels within the sensor 16 and the ability of associated signal processing circuitry to covert small voltages induced in the sensing elements to orientation information with sufficient accuracy.
An off-axis encoder is shown generally at 20 in FIG. 2. In the off-axis encoder 20, a magnet ring 22 with alternating north and south poles (not shown separately) may be mounted on a shaft (not shown for clarity of the illustration), with a sensor 24 mounted in close proximity to the magnet ring 22. Analysis of the magnetic field from multiple sensing elements within the sensor 24 is used to estimate the sensor 24 position with respect to the closest pair of magnetic poles (not shown separately) on the magnet ring 22.
Using the encoder 20 configured as shown in FIG. 2, rotary orientation can be measured to a resolution determined by the sensor's resolution per pole pair and the number of poles pairs on the magnet ring 22. For example, a magnet ring 22 with ten magnet pole pairs used with a sensor 24 providing 160 point resolution per pole pair, can effectively provide 1600 point resolution within a full rotation of the magnet ring 22.
The information available from an off-axis sensor 24, however, is related only to its position with respect to the closest pole pair. Information as to absolute rotational orientation of the magnet ring is not available. The off-axis encoder 20 is therefore unable to provide absolute orientation with respect to selected reference.
As a result, in order to obtain absolute rotational orientation information from an off-axis encoder, up on application of power the magnet ring 22 must first be rotated to a known reference position (i.e., “homed”), and the encoder reading recorded in memory. Then all subsequent encoder readings must be referenced against the reference position recorded in memory. The foregoing process is tedious. Further, in case the magnet ring rotational orientation changes during a subsequent power loss or power off condition, the change in orientation may not be recorded. Therefore, upon power up, orientation detected by the sensor 24 may be in error, necessitating another homing operation.
There is a need for a rotary encoder having high angular resolution and absolute rotational orientation information available from a single device.